Postman - The Collaboration Platform for API Development
HTTrack - HTTrack is a free (GPL, libre/free software) and easy-to-use offline browser utility.
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
WebCopy - Cyotek WebCopy is a free tool for copying full or partial websites locally onto your harddisk for offline viewing.
TestComplete - TestComplete Desktop, Web, and Mobile helps you create repeatable and accurate automated tests across multiple devices, platforms, and environments easily and quickly.
SiteSucker - SiteSucker is a Macintosh application that automatically downloads Web sites from the Internet.